1. Field of the Invention
The present invention relates to silver halide photographic emulsions which are spectrally sensitized and particularly those which are supersensitized with a combination of at least two kinds of sensitizing dyes.
2. Description of the Prior Art
A wide variety of sensitizing dyes are known to be useful for the purpose of spectral sensitization of silver halide photographic emulsions. In a specific wavelength region ranging over relatively shorter wavelengths, e.g., from the blue to green wavelength region, various kinds of sensitizing dyes are available. Especially, merocyanine dyes are often employed for this purpose. However, spectral sensitization with merocyanine dyes can occasionally result in unsatisfactory characteristics in the silver halide photographic emulsions with respect to a wide variety of purposes thereof.
For photographic materials for plate making, and particularly the so-called litho-films wherein the formation of half-tone dots is achieved by the combined use of an original having a continuous tone and a contact screen, merocyanine dyes are known to be effectively used with the intention of sensitizing these materials in the blue and green wavelength regions. As disclosed in, for example, German Patent Application (OLS) No. 2,239,711, merocyanine dyes have been employed for the sensitization of silver halide emulsions for producing lithographic light-sensitive materials since merocyanine dyes reduce the gradation thereof only slightly, and particularly, simple merocyanine dyes can sensitize these materials in the blue wavelength region, while dimethinemerocyanine dyes can sensitize these materials in both the blue and green wavelength regions. However, merocyanine dyes alone are insufficient to obtain silver halide emulsions for "lithographic" type light-sensitive materials having an additional high sensitivity in the green wavelength region, while keeping the high contrast thereof. An X-ray recording material contains sometimes an intensifying screen or fluorescent screen together with silver halide photographic film in order to enhance the sensitivity to X-ray recording. In order to minimize the amount of exposure to X-rays which are harmful to the human body, many devices for increasing the sensitivity to X-ray recording have been proposed. For example, a system using an X-ray image intensifier, a system using a solid state light amplifier and so on have been developed. In all cases, fluorescent images are recorded on silver halide photosensitive materials in the final process. Phosphors which can be employed for this purpose include blue phosphors such as calcium tungstate, zinc sulfate activated with silver, barium sulfate activated with silver, etc., and green phosphors such as cadmium zinc sulfide activated with silver, etc. In conventional X-ray recording sensitive materials, substantially all of these materials can be roughly divided into two types; one type is a regular type in which the sensitive region inherent in a silver halide emulsion is utilized, and the other type includes those materials which are spectrally sensitized orthochromaticly. The characteristics of these materials can be attributed to both the convenience in treating X-ray recording sensitive materials under as bright a safelight as is possible, and the illumination energy maximum of conventional phosphors lying in a relatively short wavelength region (about 540 - 550 nm on the longer side). However, materials which are highly sensitized to fluorescent light even if safelight safety is sacrificed to increase the sensitivity have recently been required in order to reduce even more the amount of exposure to X-rays. Spectral sensitization with merocyanine dyes alone does not satisfy very well the above-described requirements.
Spectral sensitization with known merocyanine dyes is still sufficient to satisfy the other requirements based on the characteristics of photographic materials other than the above-described characteristics, for example, reciprocity law failure caused by high intensity illumination, sensitization reduction with the lapse of time, color remaining after processing, reduction in green-sensitivity in the presence of couplers in case of color photographic emulsions, and so on.